I found that the Z-Rod coupler for the Prusa Mendel didn't hold well so went back to the drawing board.
Wanting to be done with it and move on to printing with the Prusa ASAP I took the existing "608 Ball Chain Idler with matching Pulley Hack" and 'differenced' out the 'pulley' portion and exported the .stl
Then Changed the shaft diameter to 8mm (for the zRod) and output that to .stl, Flipped it over, and combined then in Rhino. (one stl object on top of the another effectively bonds them as one object).
I apologize that it's such a hack job but it prints (and hopefully works great) and got the job done.
Print 2 (one for each stepper)
You'll need 6 ea. of M3 nuts for the captive nuts and 6 ea. M3 x 8mm grub screws. For a total of 12each.
Then I used 9,10,& 11 numbered drill bits to drill them out on the stepper end to get the tightest fit possible. I guess if you wanted you could tap the 8mm zrod end or just drill it out to 8mm (or 5/16).
P.S. the numbered drill bits where something like $2ea at the local hardware vs. $6 ea. for metric. The Grub Screws where $0.19 ea.